Quantifying sources of gas hydrate through diagenetic modeling

ABSTRACT Determining the sources of methane for hydrate formation is critical to hydrate exploration and to understanding its implications for the global carbon cycle, climate change, ocean chemistry, and biosphere throughout Earth’s history. However, quantifying the gas sources for ancient hydrate records presents a considerable challenge due to the absence of direct evidence. Authigenic carbonates (AC), formed through the anaerobic oxidation of methane (AOM), have partially retained the δ13C signature of ancient hydrates, providing a unique opportunity to identify their gas sources. In this study, we simulate the variations in δ13C values of AC precipitation associated with AOM under different AOM rates and specific methane source scenarios. Our findings suggest that the observed changes in δ13CAC with heavy δ18OAC (mostly ∼2.0‰ to 7.7‰) across global compilations associated with AOM are best explained by varying ratios of biogenic methane (BM) versus thermogenic methane (TM) and differences in AOM rates. Furthermore, our analysis indicates that TM contributions at half of the investigated gas hydrate seeps range from 17.3% to 81.5% (average 47.1%; n = 3) by volume. The compiled ages, coinciding with δ13C values of AC, suggest that amplified methane release, associated with an increased contribution from TM at high AOM rates, preferentially occurred during interglacial periods over the past 150,000 yr. The previously underestimated role of TM, and its likely positive feedback in global warming, calls for a reevaluation of the impact of methane seeps on Earth’s carbon cycle and climate through time.